The present invention relates to imaging apparatus, and more particularly relates to high-definition electronic still imaging apparatus having a liquid crystal view finder and also to imaging apparatus capable of both movie taking and high-definition still image taking.
Because of the rapid spreading of personal computers in recent years, there is an increasing demand for digital cameras (electronic still camera) as an image input equipment. Also, high-quality image recording apparatus such as digital camcorder are widely used as a recording equipment of movies. Among the several factors by which an image quality of the above described electronic still camera is determined, the number of pixels of image sensor is a major factor for determining the resolution of taken image. For this reason, even the electronic still camera of some types having a large pixel number of 4,000,000 or more are sold in the recent market.
The original purpose of the digital camera (electronic still camera) is to take still images, and it is provided with either a conventional optical finder or an electronic view finder (EVF) as the finder for confirming the extent of object to be photographed in a similar manner as the finder of the film camera. The optical finders generally have higher image quality as compared to the EVFs.
The optical finders, however, cannot follow optical zoom at a high magnification of the order of 4 to 10, and it is a general technique to use an EVF. An EVF has the resolution that is 1/10 to 1/60 of that of the image sensor of an imaging apparatus (ex. An EVF has quarter VGA: 75,000 pixels and image sensor has 3.2 M pixels. The ratio is 1/42). Considering the operation clock of pixels, it is generally difficult for an image sensor having several-million pixels to take 30 frames per second as at NTSC rate (For Example 5 M pixel picture is limited to take 8 frames per second for a pixel clock of 40 MHz. However this is the theoretical upper limit. In actual case frame number is less than 8). For this reason, a displaying based on readout of all the pixels of the image sensor having several million pixels is very difficult or costly, and only the pixels of a number required for a displaying at EVF are read out (thinned-out readout or averaging readout) so as to effect a real-time display.
Here, since the purpose of an EVF is to allow a real-time check of the situation and composition of the object, it must provide the user with a quality image. Further, there are many types of digital camera that are sold in the market as having, in addition to the view finder, a function capable also of taking movies. Conversely, a function capable of taking high-definition still images is provided also in digital camcorders. In the former case, due to the standard of image that can be recorded, an image sensor having a several-million pixels is used and the image size is reduced to VGA (about 300,000 pixels), QVGA (about 75,000 pixels) so as to effect the standard data compression based on JPEG, MPEG, etc. From the user's viewpoint, this is a result of the natural wants for freedom from the annoying situation where one has to act with having a plural type of digital camera or digital camcorder.
Because of the demands as described, the displaying/recording of a low-resolution but high-quality image is required even in those imaging apparatus having a high-resolution characteristic. Generally, lack of resolution is known to cause moire due to folding of a texture having a higher frequency than the sampling frequency. Therefore, in a known effective method for suppressing moire, all pixels of a high-resolution image are read out and is averaged in a certain area by a low-pass filter, and followed by reducing image size to a low-resolution image so that the moire is made less conspicuous by restraining band of the low-resolution image.
However, it is difficult to read all of the pixels to obtain a high-quality movie because of limitations on the frame rate of movies and on the operation clock of an image sensor as described above.
Among averaging read methods of a CCD image sensor is a method in which current values are accumulated and read out so as to increase the operation time for readout. In using this method, however, there is a problem that an occurrence of capacity overflow of transfer area causes a degradation of signals. Further, as a method of solving such a problem of addition mode readout in CCD image sensor, Japanese patent application laid-open Hei-6-217201 discloses an averaging of voltage readouts by a capacitor device of CMOS image sensor. Furthermore, a suggestion is made in Japanese patent application laid-open Hei-8-52904 where switching of averaging and thinned-out sampling is made between movie and still images, showing an advantage of improved read speed.